This invention relates to a method for the manufacture of synthetic aggregate, and the aggregate itself. In particular, the present invention relates to synthetic aggregate made from Class C fly ash.
"Aggregate," as referred to herein, is a term which includes natural aggregate, such as sand, gravel and crushed stone, as well as synthetic aggregate of various kinds, including lightweight aggregate such as cinders and aggregate produced through the combustion of pulverized powdered coal. Generally, aggregate may be used in a variety of construction and landscaping applications. For example, aggregate is an essential ingredient in most concrete formulations, including both pre-cast and cast-in-place concrete. Aggregate is also used as a foundation material for paving, and as a backfill material. Aggregate is also used in landscaping applications in both a decorative and functional manner.
When used as a construction material, aggregate is frequently required to meet certain requirements relating to its physical and chemical characteristics. Those characteristics indicate the level of quality to which it conforms. Durability is one of the most significant elements of quality in aggregate. It is determined by tests which evaluate the aggregate's susceptibility to various types of degradation. A generally recognized industry standard for evaluating the ability of an aggregate to withstand abrasion is the test known as the "Los Angeles Abrasion Test," formally referred to as the "Resistance To Abrasion of Small Size Coarse Aggregate by Use of the Los Angeles Machine"--AASHTO Designation: T 96-83 (ASTM Designation: C 131-81). The Los Angeles Abrasion Test for smaller aggregate (less than 1.5 inches) is a measure of degradation of mineral aggregates of standard gradings resulting from the combination of actions including abrasion or attrition, impact, and grinding in a rotating steel drum containing a specified number of steel spheres, the number depending upon the grading of the test sample. As the drum rotates, a shelf plate picks up the sample and the steel spheres, carrying them around until they are dropped to the opposite side of the drum, creating an impact-crushing effect. The contents then roll within the drum with an abrading and grinding action until the shelf plate impacts and the cycle is repeated. After the prescribed number of revolutions, the contents are removed from the drum and the aggregate portion is sieved to measure the degradation as a percent loss.
The present invention relates to the use of fly ash as the main ingredient in the manufacture of a synthetic aggregate. Fly ash is the finely divided residue resulting from the combustion of ground or powdered coal, such as that which is produced in electricity generating power plants. It is collected as a fine particulate from the combustion gases before they are discharged into the atmosphere. Generally, there are two classes of fly ash, the composition of which is dependent upon the composition of the original combustible material from which it is derived. Class F fly ash is normally produced from anthracite or bituminous coal, and has pozzolanic properties, i.e. little or no cementitious value, but capable of chemically reacting with calcium hydroxide to form compounds possessing cementitious properties. Class C fly ash is normally produced from lignite or sub-bituminous coal, and has some cementitious properties, in addition to pozzolanic properties.
Class C fly ash is known to be highly reactive when mixed with water, and flash setting (setting which occurs in about 2 minutes or less) will generally occur unless a reaction sequestering admixture is utilized. Use of Class C fly ash in the manufacture of lightweight aggregate and lightweight cement formulations has been suggested, for example, in U.S. Pat. Nos. 4,624,711, and 4,741,782, both to Styron, and 4,659,385 to Costopoluos et al. Styron suggests the use of Class C fly ash along with a foaming agent and an accelerator to make lightweight aggregate in a pelletizing process. Depending upon the type of fly ash used, Styron also recommends the addition of Portland cement. Costopoluos suggests the use of Class C fly ash to make a lightweight building material. Costopoluos suggests the use of air entraining and chemical foaming agents. The resulting lightweight aggregate will not conform to durability specifications for freeze-thaw and resistance to abrasion.
Generally, the prior art suggests the use of Class C fly ash only in the context of the manufacture of lightweight aggregate and lightweight cement products. The use of Class C fly ash is only mentioned in combination with foaming agents or other additives to decrease the density of the resulting product. In contrast, this invention focuses on increasing the density of material containing hydrated Class C fly ash.
It is an object of the present invention to produce an aggregate which can be used in place of standard natural aggregate.
It is another object of the present invention to produce an aggregate which is both strong and durable.
Another object of the present invention is to produce an aggregate which conforms to conventional aggregate specifications.
Yet another object of the present invention is to produce an aggregate which is relatively simple and economical to manufacture.
A further object of the present invention is to produce an aggregate which does not require expensive additives or complicated formulations.
Yet a further object of the present invention is to provide a method for making an aggregate which is simple and economical.
Still another object of the present invention is to provide an aggregate making method which efficiently and effectively utilizes large quantities of Class C fly ash to make a non-lightweight aggregate.
And another object of the present invention is to produce an aggregate with off season and non-marketable fly ash production.
These and other objects of the present invention are achieved by the practice of an aggregate making method whereby dry Class C fly ash is metered from a container. It then enters a mixing device where it is sprayed with a fine mist of water droplets. The rate at which the fly ash is metered from the container, and the rate of the addition of water is sufficient to produce a soil-like mixture. It is then distributed and uniformly consolidated, preferably by mechanical compaction, to densify the mixture. A relative density of greater than 95% of AASHTO T-180 method A modified to three lifts is desired. The densified mixture is then allowed to cure to a hardened mass. Once hardened and after a predetermined minimum time, the material is then broken or crushed into appropriate sized aggregate conforming to specific gradations established for conventional crushed aggregates. In a preferred embodiment, the compaction or consolidation of the mixture is completed before the mixture has had an initial set. In some cases, depending upon the composition of the fly ash and the amount of water added, the addition of a reaction sequestering admixture can be used to facilitate completion of the consolidation step prior to an initial set in order to achieve maximum consolidation. Depending upon the amount of water added to the fly ash, the use of vibration as a consolidation technique may be used. Where lower water content is desired for purposes of strength, mechanical compaction, as opposed to vibration, is the preferred consolidation technique.
The objects and advantages of the invention will be better understood by reading the following specification in conjunction with the attached tables and figures.